Going back a screen in the mail app is as simple as swiping right. I don't understand why you'd need a huge thumb for that. It doesn't need to reach up to the top of the screen.
The swipe gestures are not standard across all apps (they didn't seem to work in GMail, for example), and when they did I had to hit the left edge of the screen. I do not have monsterhands that can securely grip the phone, reach all the way across, and do that edge swipe. (I can hold securely the Z3c and hit almost any point on the screen with my thumb.) I also don't really like gestures--other than Swype keyboards--when using a phone one-handed, which doesn't help either.
That wasn't the main reason I took it back, though if they had an iPhone 5-sized upgraded iPhone 6 I'd probably have kept it longer. (I probably wouldn't have kept it, there were enough other frustrations that not paying $900 for a phone and being able to get a better phone for my use case for $375 was appealing, but it might have lasted longer than a snowy weekend.)
That wasn't the main reason I took it back, though if they had an iPhone 5-sized upgraded iPhone 6 I'd probably have kept it longer. (I probably wouldn't have kept it, there were enough other frustrations that not paying $900 for a phone and being able to get a better phone for my use case for $375 was appealing, but it might have lasted longer than a snowy weekend.)